[3]: 54  By the time the Commonwealth Avenue segments opened, streetcars continued along heavily congested tracks on Tremont Street (electrified in 1891) to reach the northern railroad terminals.

[3]: 39  The next month, two men were charged with a planned "conspiracy to blow up the tracks of the Elevated Company in Commonwealth av" using ten sticks of stolen dynamite.

The BERy opened a prepayment surface station (where riders paid their fares at the stop, rather than on board the streetcar) at Kenmore Street in Governors Square in 1915.

[19]: 108  The loop was heavily used during games; for the 1948 World Series, streetcars ran between Park Street and Braves Field on 45-second headways.

[19]: 102 In June 1922, the BERy proposed to operate the inner part of the Commonwealth Avenue line as a rapid transit service.

[23][3]: 59  A terminal stations would be built at Linden Street (near Harvard Avenue) in Allston, where passengers would transfer between the subway trains and surface streetcar lines.

[23] The Lechmere Square in East Cambridge (already under construction) opened that July, but local opposition to the forced transfer caused the Linden Street terminal plan to be scrapped.

In May 1924, the state legislature directed the Metropolitan District Commission to plan an expanded rapid transit system in Boston, including an extension of the Boylston street Subway under Governors Square.

[24] The report, released in December 1926, called for the existing streetcar tunnels in Boston to be reorganized into two rapid transit lines with high-floor rolling stock.

[25]: VI  Several busy grade crossings also were to be eliminated, and the report noted that the then-rapid growth along Commonwealth Avenue in Brighton might later justify extension of rapid transit to Lake Street.

[26] The tunnel was eventually chosen but construction did not begin until 1930, after the legislature lowered the cost that the BERy would pay to rent the subway from the city.

[27][3]: 44 Interest in converting the Commonwealth Avenue line to rapid transit declined as focus shifted to expanding the subway to further suburbs.

[30][31] On June 30, 1931, the existing crossover west of Blandford Street was replaced by a pocket track, allowing temporary storage of streetcars there.

[19]: 100  The short length of trackage on Chestnut Hill Avenue – which had not been used in revenue service since 1926 – was modified as part of trackwork related to the introduction of the PCCs.

[34][35]: 45  On November 25, 1961, the Boston College was cut back to Park Street station, while the 1959-opened Riverside Line was extended to Lechmere in its stead.

As part of systemwide rebranding efforts, the remaining streetcar routes feeding the Tremont Street subway became the Green Line on August 26, 1965.

[17] Over the next two decades, the downtown termini were frequently changed; the B branch variously terminated at Park Street, Government Center, Haymarket, North Station, and Lechmere.

[17] On June 21, 1969, the A branch was replaced with Watertown–Kenmore buses, halving streetcar service on Commonwealth Avenue east of Packard's Corner.

[35] The downtown terminus was extended to North Station on June 25, 2004, but cut back to Government Center on January 1, 2005.

[17] The B branch was re-extended to Government Center on October 24, 2021, as part of changes in preparation for the opening of the Green Line Extension the next year.

[40] B branch service was replaced by buses from June 20 to July 1, 2022, to allow for trackwork and installation of train protection system equipment.

In the early 2000s, the MBTA modified key surface stops with raised platforms as part of the Light Rail Accessibility Program.

The B branch is frequently criticized by riders for its slow service, with a high number of stops and level crossings.

[46] After a public comment period, Chiswick Road was removed from the proposal, as it served a nearby elderly housing community.

[51] Buses replaced rail service between Washington Street and Kenmore from April 17–May 9 and May 17–June 13, 2021, for construction of the platforms and canopy steelwork.

[58] In May 2024, the Federal Transit Administration awarded the MBTA $67 million to construct accessible platforms at the 14 B and C branch stops.

[61] That full project of station consolidation and renovations, as well as track realignment and traction power improvements, is expected to cost $221 million.

[62]: 47  A $29.3 million reconstruction and expansion of Lake Street Yard, including relocation of Boston College station, to support new Type 10 LRVs is planned for the late 2020s.